Diffused light reflectance readhead

ABSTRACT

A diffused light reflectance readhead is disclosed. The readhead employs an improved light emitting diode (LED) providing a more strongly collimated beam of light around a beam axis onto a reagent test pad. The reagent test pad is supported on a strip guide at an angle α of 5 degrees with respect to the perpendicular of the beam axis. It has been discovered that when α is between 3 and 8 degrees that specular reflection is dramatically reduced in relation to the small reduction in reflected light received by a sensor. The diffuse reflected light travels to the sensor by passing through a staircase optical baffle at an angle of 45 degrees to the perpendicular of the beam axis. The sensor converts the optical signal into an electrical one for processing and analysis. One embodiment of the present invention can detect the presence of glucose in a blood sample.

FIELD OF THE INVENTION

The present invention generally relates to the field of medicaldiagnostic equipment used in clinical chemistry. More particularly, thepresent invention relates to a improved diffused light reflectancereadhead used as part of a visual imaging system for detecting analytespresent in other substances, such as glucose in blood, on a reagent teststrip.

BACKGROUND OF THE INVENTION

Reagent test strips are widely used in clinical chemistry. A reagenttest strip usually has one or more test areas (pads), and each pad iscapable of undergoing a color change in response to contact with ananalyte in a liquid specimen. The liquid specimen is reacted with a padon the reagent strip in order to ascertain the presence of one or moreanalytes, i.e., constituents or properties of interest, in the liquidspecimen. The presence and concentrations of these analytes in thespecimen are indicated by a color change in the pads of the test stripwhen reacted with the analyte. Diffuse light reflected off of thereacted reagent test strip is analyzed to determine the amount of colorchange. Usually, this analysis involves a color comparison between thereacted test pad and a color standard or scale. In this way, reagenttest strips assist medical personnel in diagnosing the existence ofdiseases and other health problems.

An example of a reagent test strip suitable for use with the presentinvention is the Glucometer Encore®--Blood Glucose Test Strips sold byBayer Corporation, Diagnostics Division, of Elkhart, Ind. 46515.

Reflected light comparisons made with the naked eye can lead toimprecise measurement. Today, reagent strip reading instruments existthat employ reflectance photometry for reading test strip changes. Somereagent strip reading instruments have readheads that contain lightemitting diodes (LEDs) for illuminating reagent pads. Some of the lightfrom the LED is reflected off of each pad while some is absorbed in sucha way to indicate the color change of the pad due to its reaction withthe substance of interest, such as glucose. The diffuse reflected light,i.e., the color-changed light, is detected by a sensor which convertsthe light into electronic signals for processing.

It has been found that present light emitting diodes (LEDs) are notideal for use in readheads because the beam of light they produce is notvery well collimated. A significant percentage of the diffused lightproduced by present LEDs tends to become stray light that must befiltered out.

Some prior art inventions have tried to address the problem of straylight being emitted from the LED. One approach has been to encapsulatethe sides of the LED with a light absorbing material. An example of adevice with such encapsulation is U.S. Pat. No. 5,122,943 by Pugh. Thisapproach results in an LED that absorbs a portion of the light itgenerates in the encapsulation material.

It would be desirable to have an LED adapted for use in a readhead suchthat less stray light is produced that requires filtering. Moreover, itwould be even more desirable to collimate more of the light that wouldotherwise become stray light in order to increase the signal andefficiency of the readhead.

However, even when light is fairly well collimated the problem ofspecular reflection effectively raises the level of "noise" in the lightsignal received by the sensor. Specular reflection of light is analogousto light bouncing off of a mirror wherein the overall color of thereflected light is not significantly changed. Thus, specular reflectionworks against sensing a color change of a pad on the reagent strip. Itwould be desirable to decrease the specular reflection of light receivedby the light sensor in order to provide a better signal-to-noise ratio.

Because stray light makes sensing the color change of a pad moredifficult and less accurate, various optical baffles have been employedto filter some of the stray light. For example, a spiral threadedaperture has been used to reduce stray light. Only light coming from anarrow field of view can travel through the threaded aperture to thesensor, thus stray light is reduced. However, threaded apertures can becostly to form because they require extra manufacturing steps. One waythe threaded aperture is formed is by embedding a screw-like elementinto the plastic as it is being molded. When the plastic has cooled thescrew-like element is unscrewed in order to leave a correspondingthreaded aperture. Another threaded aperture drawback is that threadedapertures tend to have smaller diameters which reduce the total amountof light received by the sensor, which in turn impacts the sensor'saccuracy. Thus, it would be desirable to have an optical baffle thatreduces stray light, but is less expensive and easier to fabricate.Furthermore, it is desirable to have an optical baffle that increasesthe amount of desirable light received by the sensor.

SUMMARY OF THE INVENTION

The present invention is an apparatus and method for providing improveddetection of analytes reacted with reagent test pads. One embodiment ofthe present invention provides an improved diffused light reflectancereadhead used in a neonatal station as part of a visual imaging systemused to detect glucose in blood samples. The visual imaging systemanalyzes a color change associated with one or more test pad areas on areagent test strip following contact thereof with liquid specimen, forexample, blood or urine, in order to detect analytes such as glucose,protein, blood, ketones, bilirubin, urobilinogen, nitrite, cholesterol,etc. Light reflected off of the reagent strip is converted intoelectrical signals that can be analyzed with diagnostic equipment. Morespecifically, one embodiment of the present invention employs a newlight emitting diode (LED) optimized for use with a readhead. The LED'sgeometry has been redesigned to increase the production of collimatedlight. The length between a light emitting semiconductor and a curvedouter surface of the LED has been increased to more nearly bring thelight emitting semiconductor to a focal point of the curved outer LEDsurface. Repositioning the light emitting semiconductor in this way hasthe effect of much more strongly collimating the emitted light, therebyreducing unwanted stray light while increasing the desirableillumination of a given reagent test pad. The present design of the LEDhas the effect of improving the signal-to-noise ratio of the lightreceived by the sensor.

The reagent test strip itself is placed against a supporting surface.The surface has been tilted 5 degrees away from a plane perpendicular tothe axis of the collimated beam and away from the sensor. The small 5degree tilt has the unexpectedly large effect of reducing specularreflection to the sensor by approximately a factor of 3, dramaticallyincreasing the signal-to-noise ratio to the sensor.

To further enhance the sensor's reception of diffuse reflected (colorchanged) light a series of steps in an aperture creating a staircaseoptical baffle is employed on only one side of an aperture permittingreflected light to encounter the sensor. It has been discovered thatmost of the undesirable light entered the baffle aperture from the sidenearest the LED, where steps are now positioned, thus eliminating mostof the undesirable light. The staircase baffle reduces stray light whileallowing the rest of the aperture to be larger, thus allowing morediffuse reflected light to reach the sensor. The other sides of thestaircase optical baffle are accordingly less modified so the baffle iseasier to manufacturer in this way as well. Therefore, the staircasebaffle is both easier and less costly to manufacture.

Reflected light received by a sensor is converted into electricalsignals for processing. Analysis of the electronic signals is performedto determine the presence of glucose in blood. The present inventionprovides improved cost, manufacturing and performance advantages overcurrent systems.

BRIEF DESCRIPTION OF THE DRAWING

Other aspects and advantages of the invention will become apparent uponreading the following detailed description and upon reference to theaccompanying drawings, in which:

FIG. 1 is a block diagram overview of a diffused light reflectancereadhead according to one embodiment of the present invention.

FIG. 2 is a top view of a diffused light reflectance readhead accordingto one embodiment of the present invention.

FIG. 3 is a bottom plan view of a diffused light reflectance readheadaccording to one embodiment of the present invention.

FIG. 4 is a isometric plan view of a diffused light reflectance readheadaccording to one embodiment of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

While the invention is susceptible to various modifications andalternative forms, a number of specific embodiments thereof have beenshown by way of example in the drawings and will be described in detailherein. It should be understood, however, that this is not intended tolimit the invention to the particular forms disclosed. On the contrary,the intention is to cover all modifications, equivalents andalternatives falling within the spirit and scope of the invention asdefined by the appended claims.

One embodiment of the present invention is used in an neonatal medicaldiagnostic instrument to measure diffused light reflected from reagentpaper that has been reacted with specimen containing an analyte, such asblood containing glucose.

In FIGS. 1-4, a diffused light reflectance readhead 10 is designed withone or more improved light emitting diodes (LEDs) 12 to reflect light 13off of a reagent test pad reacted with an analyte 14. The LEDs 12 arepulsed on and off using a constant-current pulsed direct current (DC)power supply (not shown). Pulsing the LEDs 12 minimizes heating as wellas associated light 13 intensity and wavelength variation.

It has been discovered that collimated light is desirable for analysispurposes, while uncollimated light is not desirable because it tends toproduce stray light. Furthermore, it was aim discovered that the LED's12 curved outer surface in its epoxy casing acts as a lens to somedegree. The present invention takes advantage of the lensing effect byrelocating a light emitting semiconductor die 15 (also commonly known asa "chip") inside the LED 12 to a position approximately at the focalpoint of the curved outer surface. Computer modeling and experimentalresults were used to obtain an optimum tip-to-die distance (λ) of0.170±0.01 inch for this configuration. Note that other distances for λcan be used but are not considered optimal.

The present invention reduces the light's 13 resultant illumination spotsize on the reagent strip 14. A significant portion of the spot size isless than 0.100 inch in diameter at a distance of 0.150 inch beyond thereadhead surface. Because the spot size is reduced the need for afocusing lens is eliminated, thus saving its cost.

Another advantage of the present LED 12 design is that it decreases thereadhead's sensitivity to mechanical vibrations and die 15 centeringerrors. A standard T1 LED, for example, has a die-to-tip distance (λ) ofabout 0.100 inch. As λ is increased to approach the focal point of thestandard T1 LED the light 13 out from the standard LED becomes morecollimated. This tends to produce a smaller spot for an equivalentaperture size. The smaller spot size and increased collimation has theadvantage of making the readhead less sensitive to positioning of thedie 15 within the LEDs 12. Thus, if the LEDs' 12 die 15 is not placed atthe center of the LEDs 12, the output spot position will be shifted asmaller amount in proportion to the die 15 centering error.

Other factors were considered in the design of the LEDs 12. Each LED 12and each illumination aperture 16 associated with that LED 12 mustilluminate a spot of the proper size and intensity. The significantportion of the spot size should be less than the pad size to reducegeneration of stray light. Generally, greater intensity is desirablebecause signal strength is increased. Furthermore, the LEDs 12 havetheir sides coated with a light absorbing material to further reducestray light as is known in the arts. The total effect is approximately a200-300 percent signal improvement over prior LEDs used in readheads.

Light 13 from the LEDs 12 travel through the illumination apertures 16to the reagent pad 14 on a reagent test strip guide 18. It is known tohave the strip guide 18 hold the reagent strip 14 perpendicular to theaxis of the collimated light 13 emitted from the LED 12. Note that inFIG. 1 α defines the angle between a perpendicular plane 20 that isperpendicular to the axis of the collimated beam and a guide plane 22that is parallel to the orientation of the strip guide 18, which is zerodegrees in the prior art. It has been found that tilting the strip guide18 with the associated reagent pad 14 in a direction away from a lightsensor 24 by only 5 degrees, i.e., α is equal to 5 degrees, produced theunexpected benefit of reducing specular reflection received by thesensor 24 by approximately a factor of three. It was unexpected thatsuch a small change in angle α would produce such a large decrease inspecular reflection. Furthermore, the large reduction of specularreflection enabled the LEDs 12 and the light sensor 24 to be located incloser proximity to one another than currently possible, therebyreducing the size of the reflectance photometer. In one embodiment thesample to detector distance is approximately 0.4 inch, which is one halfof some current readheads, thus providing an increase in signal by abouta factor of 4. Moreover, the reduction in spectral reflection enabledthe viewing area of the light sensor 24 to be opened up, engenderingsignificant improvements in the readhead's 10 sensitivity to variationsin reagent strip 14 height variations.

The optimal range of angle a has been found to approximately rangebetween 3 degrees and 8 degrees for reagent test strips 14 reacted withblood containing glucose, however, the range should be similar for otheranalytes as well. When angle α becomes less than 3 degrees the reductionin specular reflection becomes relatively small. Conversely, when angleα becomes greater than 8 degrees then desirable diffuse color reflectionis reduced, along with undesirable specular reflection, to the pointthat significant signal loss begins to occur. Note that if α is lessthan 3 degrees or greater than 8 degrees the readhead 10 will stillperform, however, not optimally.

In FIG. 1, after light 13 from the LEDs 12 is reflected off of thereagent pad 14 it passes through a staircase baffle 26 at an angle β ofapproximately 45 degrees to the perpendicular plane 20 before reachingthe sensor 24. As shown in FIGS. 1, 2 and 4, the staircase baffle 26 hasa series of steps 28 primarily on one side. In one embodiment eachstep's top side 30 and vertical side 32 (both steps 28 and sides 30,32are representatively marked), are approximately of equal length andpositioned at a 90 degree angle to each other. It has been found thatthe length of each step side 30,32 works best to eliminate stray lightand pass desirable diffuse reflected light 13 when sized between 0.010inch and 0.030 inch. Furthermore, step 28 size is practically limited tosomething smaller than would extend into the LEDs 12 or the illuminationapertures 16. In one embodiment the length of each step side 30,32 waschosen to be 0.020 inch. The number of steps 28 is not fixed butdesirably there are a sufficient number of steps 28 to extend the entirelength of the staircase baffle 26. The total length of the staircasebaffle 26 is a function of geometry. Manufacturing limitations set aminimum separation distance between the LEDs 12 and the light sensor 24.Knowing the minimum separation distance and the distance from the LEDs12 to the reagent test pad 14 along with the fact that the steps 28 areat an angle β of 45 degrees allows a simple calculation of the distancethe steps 28 must cover. Thus, in one embodiment there are 7 individualsteps 28 as illustrated in FIGS. 1, 2 and 4.

The steps 28 are positioned on a side of staircase baffle 26 closest tothe LEDs 12. This is because it has been found that most of the straylight enters an aperture leading to the sensor 24 on the close side andthat having the steps there filters out most of stray light.

A key advantage to the use of steps 28 for the staircase baffle 26 isthe ease in manufacture over the prior art. Steps 28 can easy be formedfrom a mold while prior art threads are not so easily formed in thereadhead 10 material.

Once reflected light 13 passes through the staircase baffle 26 itreaches the light sensor 24. Note that no transfer optics are requiredbetween the reagent pad 14 and the sensor 24 because of the designadvantages of the present invention.

Devices that can be employed as the sensor 24 include charge coupleddevices (CCDs), photocells and photodiodes. In one embodiment of thepresent invention a OPT101W-R sensor from Burr-Brown, InternationalAirport Industrial Park, 6730 South Tucson Blvd., Tucson, Ariz. 85706,is employed as the light sensor 24. The sensor 24 has an electricalresponse that is proportional to the reflected light 13 received. Theelectrical response is interpreted by processing electronics (notshown). The processing electronics convert the analog electricalresponse of the sensor 24 into digital dam. The processing electronicsalso include a microprocessor (not shown) which stores and utilizes thedigital data to calculate contrast variations indicated by the sensor24. In one embodiment, the change in color is used to determine aconcentration of glucose in a blood sample.

Thus, them has been described herein a diffused light reflectancereadhead 10.

Many modifications and variations of the invention as hereinbefore setforth can be made without departing from the spirit and scope thereofand therefore only such limitations should be imposed as are indicatedby the appended claims.

What is claimed is:
 1. A diffused light reflectance readhead fordetecting diffuse light reflected off of a reagent test pad reacted withan analyte, comprising:illuminating means, having a beam axis, forilluminating said reagent test pad; supporting means for supporting saidreagent test pad in a temporarily fixed position with respect to saidbeam axis; an exit aperture positioned adjacent said illuminating means;a staircase optical baffle positioned only on the side of said exitaperture nearest said illuminating means, said baffle rejecting straylight and passing desirable diffuse reflected light from said reagenttest pad through said aperture; and a light sensor positioned to receivesaid diffuse reflected light and capable of converting said receivedlight into corresponding electrical signals; and interpretation meansfor interpreting said corresponding electrical signals to detect thepresence of said analyte.
 2. The diffused light reflectance readhead ofclaim 1 wherein said staircase optical filter has a plurality of stepseach having a top side and a vertical side.
 3. The diffused lightreflectance readhead of claim 2 wherein said top side and said verticalside are each between 0.010 inch and 0.030 inch in length.
 4. Thediffused light reflectance readhead of claim 3 wherein said top side andsaid vertical side are approximately 0.020 inch in length.
 5. Thediffused light reflectance readhead of claim 1 wherein the readheaddetects the presence of an analyte when said reagent test pad is reactedwith a sample of body fluid.